In the float process for forming flat glass, molten glass is drawn from a melting furnace and passed to a forming chamber (or "float bath") where the molten glass is deposited onto an elongated pool of molten metal such as tin or copper or alloy thereof. There, a ribbon of glass is stretched to the desired thickness as it progresses along the elongated pool of molten metal and is then withdrawn from the forming chamber as a continuous ribbon at the exit end of the forming chamber. Because of the fluid support provided by the molten metal to the glass, glass of superior optical quality can be produced by the float process.
Unfortunately, a float glass forming chamber is not free from distortion producing effects. One such effect is the "drip" problem which is caused by dripping of molten droplets of metal or compounds thereof from the roof of the forming chamber onto the glass ribbon. Although the atmosphere within the float forming chamber is usually positively pressurized with an inert or reducing gas atmosphere, sulfur and oxygen are introduced into the chamber from the glass ribbon and from other sources, and these combine with the metal of the molten metal bath to form sulfides and oxides (e.g., tin sulfide and tin oxide) which volatilize and condense on relatively cool portions of the interior surface of the float chamber. The condensate accumulates on the structural members of the bath interior, and under certain temperature and chemical conditions will be reduced to elemental metal (e.g., tin), which eventually falls as droplets onto the glass ribbon. The impact of the metallic droplets on the soft glass ribbon produces indentations which appear as optical distortions in the final glass product. This defect is known variously as "tin drip," "crater drip," "top drip," or "tin speck."
Prior art float glass forming chambers conventionally include a large number of electrical resistance heating elements for controlling the heat within the chamber, and which extend vertically through the roof of the chamber. Electrical connections are made to the heating elements above the roof by means of a complex arrangement of bus bars and leads. Because of the hot environment it has been considered desirable to retard oxidation of the connecting means by enclosing the connection means within a large, gas-tight enclosure above the roof, known as the upper plenum, and by providing the upper plenum with a non-oxidizing atmosphere. This non-oxidizing atmosphere is generally the same as the inert or reducing atmosphere maintained in the forming chamber itself, and therefore, it is customary to continuously supply the atmosphere to the forming chamber by way of the upper plenum, whereby the incoming atmosphere tends to preserve the electrical connections by cooling them as well as chemically inhibiting their oxidation. The atmosphere passes from the plenum into the forming chamber through joints around the heating elements as well as through other joints and crevices in the roof structure. It is now believed that this practice of passing the relatively cool incoming atmosphere through the roof joints aggravates the drippage problem, and therefore it is an object of the present invention to avoid this practice.
Another drawback of the prior art use of a large upper plenum to enclose the electrical connection means is that the electrical heating elements and associated hardware are rendered nearly inaccessible. As a result, it is extremely difficult to perform maintenance or to modify the heating arrangement while the float chamber is operating.